This application claims a priority based on Japanese Patent Application No. 2000-283940 filed on Sep. 19, 2000, the entire contents of which are incorporated herein by reference for all purposes.
The present invention relates to a control of a directional antenna which is set in a base transceiver station to be used for performing radio communications with a plurality of mobile stations, more particularly to a control of an adaptive array antenna capable of changing directions and widths, (radiation angles) of a plurality of beams to radiate them.
In a mobile communication system such as a car telephone and a portable telephone, an omnidirectional antenna has been heretofore set in a base transceiver station, and a beam has been radiated from the omnidirectional antenna, whereby the mobile communication system has covered a service area called a cell which has an approximately circular shape whose center is the base transceiver station. However, an explosive increase in subscribers for several years has caused a serious problem of lack in radio resources.
To cope with this problem, a technology is proposed, in which the cell is divided into a plurality of sectors, and each sector is covered as the service area. This technology is realized by setting a directional antenna (sector antenna) in a base transceiver station. According to this technology each of beams showing a directivity is radiated from the base transceiver station to corresponding one of the sectors so that the sector narrower than the cell is covered as the service area. Therefore, radio resources can be used more effectively. Here, an adaptive array antenna has been known as the directional antenna capable of radiating the beam showing the directivity in plural species. The adaptive array antenna is composed of a plurality of antenna elements, each of which is arranged so as to be spatially apart from others. The plurality of beams can be radiated in any direction and with any width by controlling these antenna elements respectively. Furthermore, a xe2x80x9cnull xe2x80x9cbeam showing no directivity can be also radiated.
As a technology for controlling beams radiated by a directional antenna, which is set in a base transceiver station, so that a beam is directed to a mobile station with a high precision, there have been technologies disclosed in Japanese Patent Laid-Open No, 9(1997)-238105 and No. 7(1995)-87011. In the former, a directional antenna is controlled so that a beam is radiated to a direction in which a level of a signal transmitted from a mobile station is high, and thus the beam is directed to the mobile station with a high precision. In the latter, a directional antenna is controlled so that a beam is radiated to a direction in which a level of a signal transmitted from a mobile station is high; a position of each of the mobile stations is estimated based on position information and the like, and a directional antenna is controlled so as to radiate a beam to the estimated position. Thus, the beam is directed to each mobile station with high precision.
By the way, there are the following problems in the above-described technologies.
Specifically, according to the technology in which the cell is divided into the plurality of sectors, and each sector is covered as the service area, the beam showing the directivity will be radiated from the directional antenna also to the sector in which the mobile station does not exist. Accordingly, radio resources are spent for the directional beam that is not actually used for a communication with the mobile station, so that waste occurs in using the radio resources.
On the other hand, according to the technology in which the directional antenna is controlled so that the beam is radiated to the direction showing the high level of the signal transmitted from the mobile stations, depending on the width of the beam, there is a possibility that the beam does not reach a mobile station which has a low level of signal but is positioned within the service area (cell) covered by the base transceiver station. In this case, this mobile station cannot get a service. Furthermore, according to the technology in which the directional antenna is controlled so that each of the beams is radiated to the corresponding one of the mobile station, processing becomes heavy in accordance with the number of mobile stations positioned in the service area covered by the base transceiver station, and furthermore the control of the beam becomes complicated. In spite that the mobile stations move at an equal linear velocity, an angular velocity of a certain mobile station among the mobile stations which move along the circumference direction around the base transceiver station is higher as this certain mobile station is closer to the base transceiver station than others. Specifically, the mobile station positioned in the vicinity of the base transceiver station generally moves at a higher angular velocity than the mobile station positioned far away from the base transceiver station. Therefore, when a large number of mobile stations exist in the vicinity of the base transceiver station, a faster processing is required to control the directional antenna so that each mobile station can be captured by a specified directional beam among the mobile stations that are moving at fast angular velocities, resulting in an increase in cost of the base transceiver station.
The present invention was made in consideration of such circumstances. An object of the present invention is to utilize radio resources more effectively. To be concrete, an object of the present invention is to control a directional antenna so that each mobile station positioned within a service area covered by a base transceiver station can be captured by one of beams, while achieving effective utilization of radio resources. Furthermore, another object of the present invention is to make it possible to capture a large number of mobile stations by beam without demanding a high processing capability.
To achieve the foregoing objects, in the present invention, a plurality of mobile stations are classified into groups, each of which includes the closely positioned mobile stations, and a directional antenna capable of changing a direction and a width (radiation angle) of a plurality of beams respectively and radiating the plurality of beams is controlled so that an area where one group is positioned can be covered by one beam, that is, so that each beam radiated from the directional antenna can be controlled for each group.
As such directional antenna, an adaptive array antenna is given, which is composed of a plurality of antenna elements disposed so as to be spatially apart from each other.
In the present invention, since no beam is radiated to an area where the mobile station does not exist, radio resources are never consumed for a beam which is not actually used for a communication with the mobile station. Furthermore, since each beam radiated from the directional antenna is controlled in its direction and width so as to be capable of covering an area where a group to be captured is positioned, each mobile station positioned in a service area covered by the base transceiver station can be effectively captured. As described above, according to the present invention, it is possible to control the directional antenna so that each mobile station positioned in the service area covered by the base transceiver station can be captured by the corresponding one of the beams, while utilizing the radio resources effectively. Furthermore, according to the present invention, since each beam radiated from the directional antenna can be controlled for each group into which mobile stations are classified, it is possible to lower a demanded processing capability compared to a case where each beam radiated from the directional antenna is controlled for each mobile station.
In the present invention, an identification number of each group may be set based on a current identification number associated with mobile stations classified into the group concerned, and then the current identification number associated with each mobile station may be updated to the identification number set for the group into which the mobile station concerned is classified. For example, an identification number is set to a group into which more mobile stations associated with the identification number concerned are classified than other groups. Further, with respect to a group to which no identification number is set in the above process, a new identification number is set to the group. Therefore, the identification number is provided with each group. Then, the identification number associated with each mobile station is updated to the identification number set for the group into which the mobile station concerned is classified.
Then, based on a movement record of each group specified by the identification number set in the above-described manner, an area to which each group moves is estimated, and the foregoing directional antenna may be controlled for each group so that one beam can cover an area in which one group is positioned and the area to which this group moves.
With such manner, for example, it is possible to control the directional antenna so that the width of the beam is wide, when the beam is for a group of mobile stations positioned in the vicinity of the base transceiver station having the directional antenna and the mobile stations move at a comparatively high angular velocity with the base transceiver station as the origin, and so that the width of the beam is narrow, when the beam is for a group of mobile stations positioned far away from the base transceiver station and the mobile stations move at a comparatively low angular velocity with the base transceiver station as the origin. Therefore, each mobile station can be effectively captured by the beam, which is positioned in the service area covered by the base transceiver station, while controlling interference of the plurality of beams radiated from the directional antenna.